An implant overdenture system 1 is used to fix animplant overdenture 3 onto alveolar ridge 8 by the supportof implants 10. The implant overdenture3 has artificialteeth 31, a denture base 32, and magnetic assemblies 4.The implants 10 each have an implant body 11 and a keeper2. A largest circumscribed circle diameter dl in an outershape of a surface-to-be-attracted of the keeper 2 isequal to or larger than .phi.1.8 mm, and a largest diameterd2 of the implant body 11 is equal to or larger than 01.2mm and equal to or less than .phi.3.5 mm, and these largestdiameters have a relationship expressed by d1/d2 <；= 1.5.
